Evaluation of Bermuda grass ( Cynodon dactylon ( L . ) Pers . ) and Bahia grass ( Paspalum notatum Flugge ) for short-term drought tolerance targeting low-maintenance landscaping

Drought responses of turf-type Bermuda grass (Cynodon dactylon (L.) Pers.) and Bahia grass (Paspalum notatum Flugge) in relation to their growth, carbon allocation and accumulation of nonstructural carbohydrates were investigated. Seedlings were exposed to a 20 day terminal drought, followed by 20 day recovery with irrigation. Leaf dry matter was lower in water-stressed plants than control plants by the end of drought, but did not differ in roots. For both species, more carbon was allocated towards roots in response to drought. Total nonstructural carbohydrate (TNC) concentration in shoots was increased in drought-stressed plants of both species. However, root TNC concentrations in Bahia grass were decreased after drought. TNC pool sizes also showed a similar variation, hence correlated with TNC concentrations. Bahia grass maintained a better quality than Bermuda grass after the drought, having recorded a higher aesthetic score, higher relative water content and a lower electrolyte leakage. Results suggest that, both species are considerably tolerant to short-term drought though Bahia grass seems more promising.


Introduction
Bermudagrass (Cynodon dactylon (L.) Pers.) and Bahia grass (Paspalum notatum Flugge) are two popular polymorphic turf grass species having C4 photosynthetic pathway.Both species are distributed most abundantly in tropical and subtropical regions, but are sparsely distributed in temperate regions (Galiano, 1985;Tischler and Burson, 1995).These grasses often experience terminal droughts
Little information is available related to drought induced carbohydrate reserves in C4 grasses, turfgrass used in landscaping in particular.Therefore, the present study was carried out with the objectives of (i) to evaluate the morphophysiological responses of Bermuda grass and Bahia grass to terminal drought (ii) to determine the total nonstructural carbohydrate (TNC) concentrations and pool sizes of two grass species in response to drought, and (iii) to determine if resilience is associated with TNC concentrations or pools.

Plant Establishment and Stress Treatment
The study was conducted in a growth chamber with temperatures of 25±2 °C day / 15±2 °C night, and a photoperiod of 14 h.Photosynthetically active radiation (PAR) just above the turf canopy at noon, averaged 850 µmol.m -2 s -1 during the study period.Seeds of Bermuda grass and Bahia grass were sown in plastic pots (15 cmdiameter, 15 cm-deep) filled with soil (Red Yellow Podsolic: pH -6.8, bulk density -1.1 g.cm -3 ).Until the experiment begins (20 days after germination -DAG), plants were watered every 4 days to bring soil to near field capacity.Thereafter, plants were subjected to water-deficit stress by withholding water (terminal drought) or were maintained well-watered (control) for 20 days (40 DAG).Stressed plants were re-watered and maintained wellwatered for another 20 days (60 DAG).Sampling was done on 20, 40 and 60 th days.

Plant Growth Analysis
Shoot and root samples were oven dried at 70 °C for 48 hours to determine the dry mass.These values were used in calculating the shoot mass ratio (SMR; shoot mass/plant mass) and root mass ratio (RMR; root mass/plant mass).Relative growth rate (RGR) was determined using the classical approach, describing the change in natural logtransformed plant mass over time (Hunt, 1982).

Aesthetic Evaluation
Visual scores for turf quality were assigned to each species across treatments.Turf aesthetics was scored from 1 to 10, with 10 equaling ideal quality based on shoot density, colour and uniformity.

Relative Water Content
Leaf water status was determined by measuring relative water content (RWC) of young and fully expanded (YFE) leaves, calculated as follows: RWC = (FW-DW)/(TW-DW) × 100, where, FW is the fresh weight, DW is the dry weight and TW is the turgid weight of leaf tissue after being soaked in water for 5 h at room temperature (Barrs and Weatherly, 1962).

Electrolyte Leakage
Electrolyte leakage (EL) was measured using YFE leaves as described in Nayyar and Chander (2004).Leaf pieces were rinsed thrice with deionized water and placed in glass tubes containing 30 ml of deionized water.These tubes were incubated for 24 hours at room temperature and subsequently determined the electrical conductivity of the solution (EC1).Same samples were then boiled for 20 minutes followed by measuring the final electrical conductivity (EC2).The EL was obtained as follows: EL (%) = (EC1/EC2) × 100.

TNC Determination
Total non structural carbohydrates (TNC) were analyzed based on the anthrone method described by Allen et al. (1974).Shoot and root samples were hydrolyzed enzymatically with α-amylase and 0.5 ml of sample extract were added to 5 ml assay media containing 13.3 M H2SO4, 26.3 mM thiourea and 10.3 mM anthrone.The mixture was heated at 100 °C for 10 min and absorbance was read spectrophotometrically at 620 nm.TNC concentrations were determined using a calibration curve done with Dglucose as standards.The total amount of carbohydrate pools (pool = TNC conc.×dry mass of plant part analyzed) in shoots and roots were also calculated as this may be important in assessing regrowth potential of plants (Santos and Trilica, 1978).

Statistical Analysis
Results were examined by one-way analysis of variance (ANOVA) using the SAS software (SAS Institute Inc.,1989).Treatment means were compared by Fisher's least significant differences (LSD) procedure at P=0.05.Qualitative data of aesthetic evaluation were analyzed using Kruskal-Wallis test.

Growth and Allocation
Shoot and root dry biomass (DM) differed significantly (P<0.01) between two species indicating the inherent variation in plant size (Fig. 1 A and B).By the end of the drought period (day 40), shoot DM of Bermuda grass (BMD) was significantly reduced by 32 %, whereas the reduction was 15 % in Bahia grass (BHI).However, root dry biomass did not differ significantly in both species owing to drought.
C.K. Beneragama, G.D. Kapila Kumara (2018)   This paper can be downloaded online at http://ijasbt.org&http://nepjol.info/index.php/IJASBTTwenty-day terminal drought reduced the proportion of biomass allocated to shoots (reduced SMR) but increased the proportion allocated to roots (increased RMR) in both species (Fig. 2).At the end of the stress period, SMR of droughted plants decreased by 13.6 % and 7.2 % in BMD and BHI respectively in comparison to their controls.RMR of drought-stressed plants was 22 % higher than that of control plants in BMD and it was 8.6 % in BHI.During drought period, mean RGRwhole plant of stressed plants was lower (P<0.05)than that of controls in both species.However, RGRroot of control and stressed plants did not differ in BHI and BMD (Table 1).Though RGRshoot was higher in stressed plants of both BMD and BHI (1.5 fold and 6 fold respectively) after re-watering, a 3 fold reduction in RGRroot was resulted in stressed BMD plants compared to controls, while it was a 1.5 fold reduction in BHI.Nevertheless, RGRwhole plant of BMD was lower in droughted than in control plants during re-growth, and in BHI it was not different.Beneragama, G.D. Kapila Kumara (2018) Int. J. Appl. Sci. Biotechnol. Vol 6(1): 12-16 This paper can be downloaded online at http://ijasbt.org&http://nepjol.info/index.php/IJASBT

TNC Concentrations and Pools
TNC concentrations of shoots were significantly (P<0.05)higher in drought-stressed plants than controls in both species (Fig. 3A).Drought induced TNC accumulation in BHI shoots was 2.7 fold greater than that of BMD.However, drought increased the shoot TNC concentration in BMD (48.1 mg/g) but decreased in BHI (30 mg/g) compared to their controls.
TNC pools in shoots differed significantly (P<0.01) for both species between treatments (Fig. 3B).BHI showed a marked increase in shoot TNC pools owing to drought.Root TNC pool sizes had a similar pattern of change as in root TNC concentrations, but with different magnitudes.However, root pool size of stressed BHI was 3.5 fold higher than that of BMD.

Plant Quality and Tissue Injury
Under drought conditions, quality of both grasses decreased markedly (Table 2).Between 2 stressed species, BHI recorded the highest aesthetic score (8.4).Compared with the control plants, shoot RWC decreased only by 20 % in BHI after 20 day drought-stress, whereas it was decreased approximately by 30 % in BMD.Both species showed a 5fold increase in EL % as affected by drought.

Discussion
Enhanced root growth during drought stress has been considered an important adaptation mechanism in many grasses (e.g.Molyneux and Davis, 1983;Huang and Fu, 2000).However our results are contradictory to the above notion.In the present study, drought did not have any impact on absolute change in root biomass in both species.Similar results have been reported by Loreti and Oesterheld (1996) with Paspalum dilatatum.Results from this study corroborate earlier findings (Huang and Gao, 2000) in that, high biomass fraction was partitioned in to roots during drought.Comparable results were reported by Fernandez et al. (2002) with Eragrostis lehmanniana and Bouteloue eripoda, both C4 grasses.However, the increase in allocation to roots in this study was mainly due to the absolute reduction in shoot biomass.
It was well established that, during drought stress, growth is ceased before photosynthesis (i.e.growth is more sensitive) and this leads to a surfeit of carbohydrates (Bressan, 2002).
Our results with Bermuda grass are in accordance with this notion.Increase in shoot TNC concentrations and pools in both grasses in this study was comparable to the results reported previously (Busso et al., 1990: Huang and Gao, 2000: Chatterton et al., 1986).However, in present study, reduction in root TNC concentrations and pools in bahia grass was observed.This substantiates the records found in White (1973).A clear relationship was not observed between TNC levels and regrowth of both species.This may be due to the fact that, in addition to TNC concentrations or pools, many other factors affect regrowth, meristem availability and activity per se (Olsen and Richard, 1988).
Electrolyte leakage from drought stressed tissues can be used as a determinant of drought tolerance (Martin et al., 1987).Present study shows drought induced leakage of solutes in both species.Bahia grass was comparatively tolerant to drought than Bermuda grass in this aspect.
Higher RWC and aesthetic scores also suggest a lesser impact of drought on bahia grass than Bermuda grass.EL, RWC and aesthetic scores were correlated to each other in both species (correlation coefficients: EL,RWC = -0.94,RWC, aestheics = 0.89, EL, aesthetics = -0.86).
Accumulated carbohydrates during a stress are of importance to plants in three ways: to use in the regrowth, to make new structural components and for osmotic adjustment (White, 1973).In our study, accumulated carbohydrates in bermuda grass roots must have been spent mainly for recovery whereas, osmotic adjustment might be the priority in Bahia grass.
In summary, both grass species exhibited reduced performances (except root biomass) in response to drought.Bahia grass was better able to maintain its quality during drought than bermudagrass, thus can be recommended for low-maintenance landscaping efforts.

Table 1 :
Relative growth rates of Bermuda grass (BMD) and Bahia grass (BHI) during drought and during recovery.Data are the means of five replicates.

Relative growth rate (mg.g -1 day -1 ) During drought During recovery
Values in columns within species followed by different letters are significantly different at P<0.05 by Fisher's LSD.

Table 2 :
Relative water content, electrolyte leakage and aesthetic scores of bermudagrass (BMD) and Bahia grass (BHI) as affected by drought.Data are the means ± SD of five replicates